1. Field of the Invention
The present invention relates to an optical pickup head for optically recording, reproducing, or erasing information on or from an optical or magneto-optical medium such as an optical disk or an optical card.
2. Description of the Prior Art
An optical memory technology using an optical disk having pit-pattern has been expanding its utilization field as a high-density and large-capacity recording medium, so as to be used as a digital audio disk, a video disk, a floppy disk, and further a data file.
For such an optical memory technology, it is important to accurately carry out the recording on or reproducing from the optical disk through optical beam squeezed in a thin beam with high reliability. This mechanism largely depends on its optical system in reliability.
An optical head generally serves as an essential part of the optical system. Basic functions of the optical head are roughly categorized into a convergence for forming a diffraction-limited small spot, a focusing servo and tracking control in the optical system, and pit signal (information signal) detection.
These functions are embodied by various combinations of optical systems and photo-electrical conversion detecting systems on the basis of their purposes and uses. Especially, an optical pickup head apparatus using hologram has been recently introduced in order to reduce the size of optical pickup head apparatus itself and manufacture it thin.
As an example of prior art, FIGS. 24 and 25 show a constitution of an optical pickup head disclosed by J. C. LEHUREAU, J. Y. BEGUIN and J. COLINEAU; "Polarizing Grating Beamsplitter Using a Liquid Crystal Cell", Proc. Int. Symp. on Optical Memory, 1989 Japanese Journal of Applied Physics, Vol. 28 (1989) Supplement 28-3, pp. 201-203.
In FIG. 24, a reference numeral 2 denotes a radiation light source; for example, a semiconductor laser. A linearly polarized light beam 3 (a laser beam) emitted from this light source 2 passes through a liquid crystal hologram 172 and, in turn, is converted into a circular polarized beam by means of a quarter wavelength plate 15. Then, the circular polarized beam enters into an objective lens 4 and converges onto an information medium 5.
After reflection at the information medium 5, the rotational direction of the circular polarized beam is reversed. Then the light beam 3 travels the same light path in an opposite direction, and enters again into the quarter wavelength plate 15. In the quarter wavelength plate 15, the light beam 3 becomes a linearly polarized beam with a polarized direction rotated 90 degrees from its initial direction.
Subsequently, the light beam 3 enters into the liquid crystal hologram 172, and from which a +1-order diffraction light beam 66 inters into a photodetector unit 7. By calculating outputs of the photodetector unit 7, servo signals (i.e. focus error signals and tracking error signals) and information signals can be obtained.
As shown in FIG. 25, the liquid crystal hologram 172 consists of a pair of transparent substrates (glasses) 9 and 9, a polyimide 23 for forming a brazed hologram, a liquid crystal 17, and a pair of transparent electrodes 16 and 16.
An agent for orientating liquid crystal molecules and a sealing material are often used for the liquid crystal hologram 172 but are not related to the present invention. Therefore, they are not shown in the drawing. The liquid crystal 17 includes elliptic liquid crystal molecules 17a having a refractive index n.sub.s in its minor-axis direction and a refractive index n.sub.l in its major-axis direction. In this case, the refractive index n.sub.s is selected to be substantially the same as a refractive index n.sub.p of the polyimide 23 and the refractive index n.sub.l is largely different from the refractive index n.sub.p.
When a light beam 3 being linearly polarized in the minor axis direction (i.e. direction 1) of the liquid crystal 17 oriented in this liquid crystal hologram 172 is entered into the liquid crystal hologram 172, no diffraction is generated since the polyimide 23 and the liquid crystal 17 have substantially the same refractive index (n.sub.s .apprxeq.n.sub.p).
On the contrary, when a light beam 3 being linearly polarized in a different direction (i.e. direction 2) normal to the above-described polarized direction is entered into the liquid crystal hologram 172, a diffraction is generated due to a refractive index difference between the liquid crystal 17 and the polyimide 23 (n.sub.l .noteq.n.sub.p). Furthermore, since the polyimide 23 is brazed as shown in FIG. 25, +1-order diffraction light beam becomes strong.
By utilizing above-described property of such a liquid crystal hologram 172, if the light beam 3 is emitted in FIG. 24 so that a polarized direction of the light beam 3 becomes parallel with the direction 1, no diffraction occurs in a light beam path (forward light beam path) leading from the radiation light source 2 to the information medium 5 but the diffraction efficiency of the +1-order diffraction light beam becomes high in its opposite light beam path (backward light beam path) leading from the information medium 5 to the radiation light source 2.
Accordingly it is concluded that the efficiency of use of light beam in the forward and backward light beam paths is high. Hereupon, the efficiency of use of light beam in the forward and backward light beam paths is defined in general by multiplying the light quantity (0-order diffraction light quantity) passing through the hologram 172 in the forward light beam path and the +1-order diffraction light quantity emitted from the hologram in the backward light beam path.
According to this prior art, a focusing error (FE) signal is sensed by Foucault method or by astigmatic method. However, in the case where the Foucault method is adopted, a mechanical knife edge 22 shown in FIG. 24 must be additionally installed. On the other hand, in the case where the astigmatic method is adopted, a cylindrical lens must be additionally installed. In any case, there was a problem that the number of parts increased and therefore it resulted in cost up.
One method for solving above-described problem is to give a curvature on a hologram curve of the liquid crystal hologram 172. However, as is mentioned in the above-introduced reference paper, it is known that manufacturing a brazed hologram to have a curvature on its hologram pattern has been quite difficult.
Furthermore, this prior art mentions that the liquid crystal hologram can be generally used as a substitution for one polarized beamsplitter in an optical pickup head apparatus for a magneto-optical disk requiring more than two polarized beamsplitters. However, this is only effective in reducing a size of one component, and does not result in a sufficient overall reduction in size of optical pickup head apparatus.